1. Field of the Invention
The present invention relates to a beach nourish work on a shore using a bed for wave dissipation having a lattice frame and a construction method thereof.
2. Description of the Related Art
A natural shore is continuously moving. Formerly, soil was transferred from the upstream of a river to the downstream, flowed out into the sea and was washed up on a shore, returned to the sea by a backwash and transferred to various places by a coastal current. In this way, a shore kept its balance.
Recently, however, a flood-control dam and a sand-trap dam constructed on the upstream of a river, a revetment provided for river improvement and the like interrupt the supplying source of sand to the downstream, further, a port is constructed on a shore and a jetty is constructed to prevent erosion of a shore, consequently, the environment of a shore changes, a balance between supply of sand to a shore and flow out of sand from the shore kept so far collapses, and then the amount of sand carried out increases more than the amount of sand supplied. This mechanism is a main cause for shore erosion.
Therefore, to protect a shore from erosion, a wave dissipating structure formed by merely stacking wave dissipating blocks near a shore line of a shore to buffer waves coming toward the land, a sloped revetment formed by arranging flat plate blocks on a slope having a gentle gradient, a revetment composed of a vertical retaining wall to directly prevent an ocean wave or the like are constructed. However, in the case of the wave dissipating structure composed of stacked blocks, even when sheets and mats are used under the structure, the corner of a deformed block often thrusts into the sheets and mats and makes a hole because of the concentrated load, and the sheets and mats are torn. Once torn, the crack easily spreads. Further, when a wave dissipating structure is constructed first, the wave dissipating structure does not sink much owing to the effect of the sheets and mats, therefore, the sheets and mats are intensely abraded by waves containing sand, and sometimes may become ragged in only a few months. Consequently, as shown in FIG. 1, blocks 7 collapse successively, and are finally completely scattered. This phenomenon occurs of course when the sheets and mats are not used at the bottom. Then, a breaking wave 4 rushes up vigorously as shown by mark 5 in FIG. 1, a backwash carries out sand in the offing direction as shown by dotted line 6, and waves which successively rush up move as indicated by marks 5' and 5" in FIG. 1, erosion of the shore progresses as indicated by marks 6' and 6" in FIG. 1 and the shore is eroded and decomposed. Thus collapsed and scattered blocks are exposed on a natural shore as ugly wrecks, and often disturb a rare scenic area praised because of white sand and green pine trees, which we Japanese greatly appreciate.
Further, in the case of a sloped revetment, the force of a wave rushing up is not suppressed at all, and a backwash flows vigorously through the bottom of the blocks, makes sand and blast of the foundation flow out and decomposes and scatters the flat plate blocks. Also regarding the vertical retaining wall, when a heavy storm occurs, the tide level rises extraordinarily, waves colliding with the vertical retaining wall washes and scours the foundation strongly, makes the wall collapse, and sometimes cracks it apart. The present inventors have previously developed and proposed to lay a lattice bed for wave dissipation formed by placing steel products lengthwise and crosswise on the ground at the bottom of water and further to stack successively a wave dissipating material such as blocks and the like on the bed to make a wave dissipating structure for scour protection of a slope such as a breakwater, a ripraped breakwater or the like (Japanese Patent Application Publication (JP-B) Nos. 60(1985)-17888 and 8(1996)-19658).
This technique will be illustrated by a drawing. FIG. 2 shows a bed 10 for wave dissipation having a lattice frame, and mark 1 indicates longitudinal inner partitioning materials and mark 2 indicates horizontal inner partitioning materials. It is intended to develop naturally a seaside place by placing beds 10 for wave dissipation having a lattice frame (hereinafter, referred to as "lattice frame 10") near and approximately parallel to a shore line and by stacking blocks and the like on the beds to construct a wave dissipating structure.
In a conventional wave dissipating structure near a shore line, since blocks are easily scattered, blocks having a weight several times as heavy as a required weight calculated from a designed wave height are used, and due to such large blocks used, a remarkable shore scene can not be seen and the scene is often remarkably disturbed.
Further, when a block is large, even if the void ratio does not vary depending on the size of the block, the void itself increases in size and consequently a wave flows through the void without substantial suppression of wave energy, therefore, the background is often cut out and eroded.
However, since there is no definite theory regarding the sinkage of a structure placed near a shore line, there has been no construction method in which a wave dissipating structure is placed near a shore line, and sand is accumulated naturally.